[vdr] mdadm software raid5 arrays?

[jori.hamalainen at teliasonera.com]

> Ok.. short comparison, using a single disk as baseline.

Good chart, perhaps you should also mention the capacity. With this I 
mean what happens.

1 disk = 1TB for simplicity.

>using 2 disks
>raid0: (striping)
> ++   double read throughput, 
> ++   double write throughput, 
> --   half the reliability (read: only use with good backup!)
Capacity: 2TB

>raid1: (mirroring)
> ++   double read throughput.
> o    same write throughput
> ++   double the reliability
Capacity: 1TB

>using 3 disks:
>raid0: striping
> +++  tripple read performance
> +++  tripple write performance
> ---  third of reliability
Capacity: 3TB

>raid1: mirroring
> +++  tripple read performance
> o    same write throughput
> +++  tripple reliability
Capacity: 1TB

>raid5: (distributed parity)
> +++  tripple read performance
> -    lower write performance (not due to the second write but due 
>      to the necessary reads)
> +    sustains failure of any one drive in the set
Capacity: 2TB

>using 4 disks:
>raid1+0:
> ++++ four times the read performance 
> ++   double write performance
> ++   double reliability
Capacity: 2TB

Raid5:
> +++  four times the read performance
> -    lower write performance (not due to the second write but due 
>      to the necessary reads)
> +    sustains failure of any one drive in the set
Capacity: 3TB
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